Fix warning about ignored attributes for wxControlRenderer

This warning was given many times and was very confusing as it wasn't given at
all in the right location, but rather for all the other, subsequent
declarations of wxControlRenderer which were correct -- unlike this one which
used wrong order for the "class" keyword and the attribute.

include/wx/univ/scrolbar.h

@@ -191,7 +191,7 @@ private:
     // the object handling the arrows
     wxScrollArrows m_arrows;
 
-    friend WXDLLIMPEXP_CORE class wxControlRenderer; // for geometry methods
+    friend class WXDLLIMPEXP_FWD_CORE wxControlRenderer; // for geometry methods
     friend class wxStdScrollBarInputHandler; // for geometry methods
 
     wxDECLARE_EVENT_TABLE();